This invention relates to a device to simulate and indicate winding temperature in oil filled power transformers.
Large power transformers in use today work with a very high degree of efficiency. There is an energy loss in the transforming operation, however, and this loss manifests itself in the form of heat. The heat must be removed from the coils and core assembly to prevent premature aging of the insulation and subsequent failure or destruction of the transformer. The degradation of the insulation is a function of time and temperature. The most common coolants are oils which have good insulation properties, a low viscosity, a high relative thermal capacity and a good co-efficient of expansion for good circulation. Care must be taken to ensure both that there is proper oil circulation and that the oil does not overheat. Uncontrolled high oil temperatures may reduce transformer life or result in the insulation failure.
It is therefore necessary to keep a constant review of the oil temperature. Liquid temperature wells are used to measure the oil temperature along the outer wall of the transformer. They are not, however, suitable to measure the oil temperature around the transformer coils, which will be higher during use. The temperature of the core or winding can rise dangerously before the oil temperature changes are noted by the liquid temperature well on the transformer wall.
Winding temperature heater wells and winding temperature indicators have therefore been introduced. These devices simulate the amount of energy passing through the transformer at any particular time. An electrical current, proportional to the load, is taken by means of a current transformer to heat and run through a resistor in the winding temperature heater well. The heat generated by the resistor is measured by a temperature sensing bulb on a temperature detecting device. The temperature of the winding temperature indicator is therefore increased above the surrounding oil temperature proportional to the load.
Once calibrated the winding temperature heater well can be used to measure the core and winding temperature, which when combined with the wall oil temperature measured by the liquid temperature well, can accurately forecast dangerous overheating of the transformer.
There are several types of winding temperature heater wells known. They tend to be complicated in structure and/or difficult to calibrate. The general construction is to provide a temperature detecting device, a resistor element through which the current is run and a metal sleeve thereover to protect the inside workings from the oil. The resistor element comprises either a winding coil or a solid tube surrounding the temperature detecting device. It is also common to place a second sleeve about the first sleeve and fill the intervening gap with an oil to act as an insulator for the device. In known systems, the resistor often must waste heat heating the oil used as an insulator in the well. This causes calibration problems as the insulating oil affects the amount of heat measured by the heat detecting device.
The purpose of the device is to reflect the hottest spot of the winding transformer coils, by measuring the heat put out by the resistor, the amount of heat being directly determined by the amount of electrical current through the resistor element. It is therefore important that the heat generated be accurately reproduced proportional to the transformer load.
The known devices are large and bulky and so require a long warm up time till they are heated up and start radiating this heat to the temperature detecting device. The large resistors in these devices require a large current to heat them up. Access to the known devices is also often retricted. The parts are usually delicate to handle and must therefore be assembled at the time of installation.
The known winding temperature well devices are also unduly affected by ambient temperatures as they radiate heat externally which may not be fully detectable by the temperature sensing bulb. Calibration of the resistor element is then not accurately possible as not all the heat energy being radiated is being measured.
The temperature wells are usually mounted in a plate structure in a wall of the transformer. Presently, there is not known any such structure which properly electrically and thermally insulates the winding and liquid temperature wells from the ambient temperature.
Another problem with presently known wall plate structures is that the wiring from the winding temperature well to the calibrating resistor is often externally located. This requires long conductors around the transformer housing exposed to ambient temperature changes. Damage to these exterior connections and possible calibrating errors are more likely in this situation than if the connections were made inside of the wall plate structure.